Summary
Highlights
Lateral force analysis procedures are categorized into linear and non-linear, and further into static and dynamic methods. The equivalent static method (static linear), also known as the lateral force method or seismic coefficient method, is the simplest and most commonly used for low to mid-rise structures and is the focus of this discussion. Other methods include pushover (static non-linear), response spectrum and linear time history (dynamic linear), and non-linear time history, displacement-based, energy-based, and capacity-based methods (dynamic non-linear).
Dynamic non-linear analysis is the most complex type, requiring computer tools, extensive earthquake ground motion data, and is prone to errors if attempted manually due to the vast amount of data involved.
Before conducting lateral force analysis, it's crucial to identify the effective seismic weight (W). This is equivalent to the total seismic dead load. A higher seismic weight leads to a greater base shear, making lighter structures advantageous, especially for high-rise buildings. The effective seismic weight includes all dead loads, 25% of the floor live load for storage/warehouse occupancies, partition loads not less than 0.5 kN/m², and the total weight of permanent equipment.
The NSCP (National Structural Code of the Philippines) outlines three analysis procedures: simplified static, static, and dynamic. The code specifically details the static linear analysis. The simplified static method is for occupancy category 4 or 5 buildings, not exceeding three stories (light frame construction) or two stories (other buildings).
Static analysis is applicable for regular or irregular structures in occupancy categories 4 and 5 within seismic zone 2. It can be used for regular structures under 75 meters in height, provided the site location is not soil profile type SF. For irregular structures, it's limited to those not more than five stories or 20 meters in height. Many designers prefer static analysis due to its simplicity and less demanding data requirements compared to dynamic analysis.
Dynamic analysis is mandatory when simplified static or static analysis is not permitted, such as for structures over 75 meters in height, structures with irregularities (stiffness, weight, mass, geometric, vertical), structures over five stories or 20 meters in height in Zone 4 that lack a consistent structural system, or regular/irregular structures located in soil profile type SF with a period greater than 0.7 seconds.